It has become increasingly desirable to construct components for bicycles and other pedal powered vehicles having reduced weight while maintaining or enhancing component strength. Designers have paid increased attention to the pedal crank assembly that transmits power from the legs of the rider to the wheels of the bicycle. This assembly experiences extreme stress both in torsion and bending. This stress is largely cyclic and, thus, bottom bracket assemblies must retain sufficient strength to resist fatigue failure.
Designers of pedal powered vehicles have generally constructed pedal crank axles for bottom brackets as a single, approximately uniform diameter shaft. An example of such a prior art shaft is depicted generally in FIG. 1. The shaft 10 has tapered spindles 12 at either end that project from the bottom bracket (not shown). The center 14 of the shaft 10 is, as noted, of roughly similar diameter to these spindles 12 and passes through the interior of the bottom bracket. The bottom bracket supports the shaft by means of bearings (not shown) positioned in end caps or cups proximate each spindle. In general, one cup is axially fixed while the other includes a locking ring for axial adjustment. The bearings generally used are unsealed balls that are positioned about the circumference of the shaft and directly engage radiused shoulders 16 of an increased diameter section 18 machined on to each side of the shaft. In this manner, the shaft 10 is held axially and radially within the bottom bracket. Since ball bearings are positioned between the axle shaft outer diameter and inner circumference of the bottom bracket, there is a substantial difference between the inner diameter of the bottom bracket and the smaller outer diameter of the shaft.
In conventional designs of bottom bracket axles, the center portion of the solid axle shaft is of similar diameter as the outwardly positioned spindles and merely extends through the hollow central portion of the bottom bracket with substantial radial space between the shaft outer diameter and bottom bracket inner diameter. This space is largely wasted while a heavy and relatively thick central axle section is needed to provide adequate strength.
A further disadvantage of conventional bottom bracket axle designs is that they must have sufficient shaft strength to resist torsional, bending and cyclic loads. As a result such shafts, which are generally formed of steel alloy, are rather heavy.
These conventional designs also do not adapt well to modern light-weight materials such as titanium. Shafts constructed in the conventional manner (with a conventional narrow central section) from titanium have proven flexible or whip like under torsional loading and their shoulders have experienced undue wear in contact with ball bearings.